Homogeneous oxidation of SO2 in the tail gas incinerator of sulfur recovery unit

The formation and emission of sulfur trioxide (SO3) in sulfur recovery unit has received increasing attention due to its adverse effects on the operation of plant and environment. Due to the excess oxygen, high concentration of SO2 and high temperature, SO3 formation in the sulfur recovery unit tail gas incinerator may significantly increase. A small horizontal tube reactor was employed to simulate the homogeneous oxidation of SO2 in the tail gas incinerator. The SO3 concentration was measured with a controlled condensation method at the outlet of the reactor. The present work focuses on the gas-phase chemistry and examines the impact of different combustion parameters and atmospheres on the formation of SO3 in the tail gas incinerator. Experiment results show that the increased O2 and SO2 concentrations along with increasing temperature are favorable for enhancing SO3 formation over the range of tested parameters. The presence of water vapor has an enhancing effect of SO2 oxidation in the experiments conducted. No significant effect of CO2 was found to the oxidation of SO2.


Introduction
The oxidation of SO 2 relate to the sulfur compounds combustion becomes an increasing concern.The high concentration of oxidation products such as sulfur trioxide (SO 3 ), which could react with water molecule form H 2 SO 4 or sulfuric acid mist, is not only a corrosion risk to equipment but also an environmental concern.In sulfur recovery unit, the tail gas stream from Claus tail gas treatment plant is predominantly N 2 , H 2 O vapor and CO 2 depending on the original acid gas.This stream will also contain a number of residual sulfur compounds such as H 2 S, SO 2 , COS, CS 2 and S vapor together with other components, such as CO and H 2 , at low concentrations.Compliance with regulatory guidelines of pollutant emission, requires the destruction of all reduced sulfur compounds in the tail gas down to low levels before allowing the stream to enter [1] the atmosphere [1,2].This is typically done by oxidizing the reduced sulfur compounds in the tail gas to SO 2 within the plant incinerator.One of the preferred methods to incinerate the Claus tail gas stream is thermal incineration, where high temperature homogeneous gas phase reactions take place for oxidation of the reduced sulfur compounds and then all the sulphur compounds are converted to SO 2 .When excess oxygen is present and gas temperature is sufficiently high, a small amount of SO 2 could be converted to SO 3 in the flue gas [3][4][5].SO 3 is a great concern in anti-corrosion design, due to the presence of SO 3 and vapor in the flue gas.Sulfuric acid can condense at very high temperature in flue gas, wherein the dew point of the sulfuric acid in the flue gas from the sulphur tail gas incinerator is typically 120-170 °C [6].The Claus tail gas and sulphur degasser off-gases are both routed to the incinerator, resulting in relatively high concentration of SO 2 in the flue gas.Due to the high concentration of O 2 , SO 2 and water vapor in the incinerator, more SO 3 tends to be formed.Higher SO 3 concentration together with a mass of water vapor in the flue gas will obviously increase the acid dew point of the flue gas, increasing the risk of corrosion on the equipment.Care must be taken to prevent the steel shell of the equipment from getting below the dew point of the acid gas [7].Beyond that, sulfuric acid in flue gas is an environmental concern, and a small amount of SO 3 emission may cause serious environmental problems.With the decrease in temperature, the H 2 SO 4 vapor condenses to form acid mist, leading to acid deposition [8], and furthermore, sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates.
Many studies have been done to explore the formation of SO 3 in combustion process by both experiment method and theoretical modeling [9][10][11][12][13][14].In principle, SO 3 formation from SO 2 oxidation during the combustion process results from either homogeneous gas reactions or heterogeneous catalytic reactions.Sulfur trioxide is thermodynamically favored at lower temperatures, but kinetic limitations and short reaction time often prevent an SO 3 / SO 2 partial equilibrium from being attained [10].From the research work of Hindiyarti et al. [15] two reaction routes are recognized as the dominant pathways in the oxidation of SO 2 in gas phase reaction.
The primary reaction is the direct reaction between SO 2 and oxygen radical at temperature higher than 900 ℃: The other reaction path occurs under moist atmosphere, where oxidation of SO 2 is strengthened in the presence of water vapor which increases the O/H radical concentration: The first reaction path is the main source of SO 3 formation at high temperature, and the second reaction path (1) contributes mostly to the generation of SO 3 under moist atmosphere at low temperature.HOSO 2 is unstable at high temperature, and production of SO 3 via HOSO 2 is insignificant at temperatures of 1000 K and above [13,16,17].
Due to the significantly increasing concentration of SO 2 and oxygen compared to air firing, lots of research works focus on the SO 3 formation in oxy-fuel circulating fluidized bed [4,14,[18][19][20][21].SO 3 concentration measurement under oxy-fuel condition also indicates an increase of SO 3 concentration in flue gas [5].Compared with the conditions in oxy-fuel combustion, because the Claus tail gas and sulphur degasser off-gases are both routed to the incinerator, the concentration of SO 2 in tail gas incinerator are around 1% vol, higher than that in oxy-fuel combustion [14,22].Since limited work has been published focusing on the generation of SO 3 in incinerators of the sulfur recovery unit, it is necessary to have a deep look at the formation of SO 3 in the tail gas incinerator of the sulfur recovery unit.
In the present work, the focus is to investigate the homogeneous gas phase SO 3 generation in the tail gas incinerator of the sulfur recovery unit under different combustion conditions.The effects of concentration of reactant gas, reaction temperature and the presence of impurities in flue gas were investigated on the formation of SO 3 .

Experimental section
The experimental setup consists of a gas feeding system, a mixing chamber, a reactor system, an acid mist condenser and a tail gas treatment unit, as shown in Fig. 1.The gas feeding system consists of gas cylinders that are connected to the reactor system with mass flow controllers from the Beijing Sevenstar Electronics Co., Ltd. to the reactor system.High purity air, CO 2 , SO 2 , N 2 (21% Oxygen in N 2 , 99.99% CO 2 , 10% SO 2 , 99.99%N 2 ,) are used to simulate the reaction gas, N 2 is the balance gas.The concentration of O 2 , SO 2 , CO 2 in the reaction gas are adjusted by controlling the flowrate of corresponding mass flow controller.The controllers are adjusted before the experiment using a bubble flow meter.All the reaction gases including water vapor are mixed in the mixing chamber.A water saturator maintained at a controlled temperature was used to introduce water vapor into the N 2 carrier gas.The amount of water vapor introduced to the reaction system is measured by the loss of weight of evaporated water.The concentration of water vapor is controlled by the flowrate of carrier gas.According to the actual condition in the tail gas incinerator of the sulfur recovery unit, the concentration of SO 2 and oxygen in the tube reactor were set to be around 1% vol and 4% vol respectively.The reactor system consists of a quartz glass tube flow reactor that is inserted for the oxidation of SO 2 [11].The tube reactor characterized with 16 mm inner diameter and 60 cm total length, is located in a one zone electrically heated horizontal furnace with an isothermal length of 18 cm.According to the residence time of flue gas in tail gas incinerator, the residence time of reaction gas in the isothermal zone was set to be 2 s at the actual reaction temperature, and the total gas flow rate into the reactor was adjusted to meet the residence time, typically 263 ml/min (@1 atm, 273 K) for the reaction temperature at 1123 K.The temperatures in the tube reactor were measured under inert atmosphere with a type K thermoelectric couple (± 5 K).The gas line behind the outlet of reactor was electrically heated with a heating tape to a temperature around 250 ℃ for the purpose of avoiding condensation [10].
To determine the oxidation of SO 2 in the reactor, the controlled condensation method (CCM) was employed to measure the concentration of SO 3 at the outlet of tube reactor.SO 3 was extremely reactive in flue gas, with the decrease of flue gas temperature, and water vapor and SO 3 beginning to form vapor sulfuric acid.When the temperature of flue gas below is 200 ℃, almost all the SO 3 is converted to H 2 SO 4 in the humid condition of typical flue gas [23].With the continuous decrease of flue gas temperature, once the temperature was below acid dew point, the gaseous H 2 SO 4 began to condense to sulfuric acid.The temperature of gas line between the outlet of the tube reactor and the inlet of the condenser was controlled around 250 ℃ to avoid condensation.The temperature of flue gas in the condenser was set around 85 ℃ which was above the dew point of water vapor to prevent the condensation of water and associated SO 2 capture by condense water, but also kept below the acid dew point which allow the condensation of sulfuric acid.With this approach, SO 3 in the flue gas was selectively condensed and the interference of high concentration of SO 2 in the flue gas is avoided.Moreover, about 0.5 g quartz wool is placed inside the condenser to improve the recovery rate of acid mist.When exploring the reaction between SO 2 and O 2 , water vapor is needed to condense the SO 3 in the flue gas, as shown in Fig. 1.A three-way valve was used at the outlet of the tube reactor for introducing water vapor into the flue gas, the same as the introducing of water vapor into the tube reactor.Furthermore, the total gas flow rate into the condenser can be adjusted through the three-way valve.All the experiments in this study, the total gas flow rate is set to be 1.5 L/min.Two measurements are conducted under one experiment condition, the average value of measurements is used as one data point.
The sulfuric acid present in the condenser was flushed by a known quantity of distilled deionized water.The conversion of SO 2 to SO 3 in the reactor was determined by the amount of H 2 SO 4 in the condenser.An ion chromatogram analyzer (ICS-1100, Dionex) was used to determine the concentration of H 2 SO 4 in the distilled deionized water.The measurement error of ion chromatogram analyzer is within 5%.
To evaluate the accuracy of the SO 3 collecting system, a SO 3 sampling system has been built in laboratory, as shown in Fig. 2, where dilute sulfuric acid has been evaporated to simulate the sulfuric acid mist in the reaction gas.In general, almost all the SO 3 convert to sulfuric acid mist in the snake condenser.Therefore, the concentration of SO 3 seem same as the concentration of sulfuric acid mist.The collecting efficiency of the condenser is defined as the ratio between the amount of sulfuric acid mist collect in the condenser and the amount of sulfuric acid introduced to the evaporator.
In the sampling experiments, the concentration of SO 2 , O 2 and CO 2 in the gas is set to be 1%, 4% and 20%, N 2 is the balance gas.The flowrate of the gas is set at 1.5 L/min, temperature of evaporator is set around 400 ℃, the temperature of heating tape and condenser is set to be 250 and 85 ℃, the flow rate of dilute acid introduced into the evaporator is set at 0.24 ml/min.The adjustment of the SO 3 concentration in the gas could be realize by changing the concentration of dilute acid introduced to the system.The concentration of dilute acid introduced into the evaporator is about 5 mM to 30 mM.The average value of two measurements under one experiment condition is used as one data point.As shown in Fig. 3, the collecting efficiency of sampling system is relatively stable with the change of SO 3 concentration, the collecting efficiency is around 90%.

Effect of SO 2 concentration
To study the effect of input SO 2 concentration on the output SO 3 concentration in the flue gas, the conversion of SO 2 to SO 3 and the order of reaction with respect to the SO 2 concentration were obtained.Experiments were conducted at temperature 850 ℃ with a fixed oxygen concentration of 4 vol% and by varying the SO 2 concentration among 0.52, 0.75, 0.87, and 1.31 vol%.
When the compositions in the reaction are far away from equilibrium, the reversible reaction can be ignored, and under such conditions, the rate of the generation of SO 3 can be expressed by the following empirical equation: where n and m are reaction orders with respect to SO 2 and O 2 concentrations.The oxidation of SO 2 was less than 2% under our experiment conditions, its concentration can be assumed to be constant (i.e.,P SO 2 = P 0 SO 2 ).Additionally, the consumed oxygen calculated from the generated SO 3 were calculated to be less than 1%.Therefore, for the sake of simplicity, if it is assumed that both the reactants were in excess compared to the product, the relationship ln(P so 3 ) = ln(k′t res ) + m ln(P 0 so 2 ) can be derived, where k´ is equal to k f times (P 0 O 2 ) n /[RT ] m+n−1 .Hence, the slope (m) of a plot of ln(P SO 3 ) versus ln(P 0 SO 2 ) would result in the order of reaction with respect to the SO 2 concentration.
In Fig. 4a, the concentration of SO 3 at the outlet of the reactor and the conversion of SO 2 is plotted against the input SO 2 concentration.SO 2 concentration varied from 5200 ppm to 13,100 ppm, corresponding to the high SO 2 concentration in the tail gas incinerator.As expected, the concentration of SO 3 was higher with the increasing SO 2 concentration in the flue gas.The concentration of SO 3 at the outlet of the reactor was measured from 24 to 60 ppm in the experiments conducted.The conversion of SO 2 to SO 3 ranged from 0.544% for 5200 ppm of SO 2 input to (4) Fig. 2 The diagram of SO 3 collecting system built in laboratory Fig. 3 The collecting efficiency of SO 3 collecting system with different SO 3 concentration 0.489% for 7548 ppm of SO 2 , 0.484% for 8747 ppm of SO 2 , and 0.429% for 13,100 ppm of SO 2 .
The oxygenation efficiency varied with the concentration of SO 2 for the SO 2 oxidation are according with previous works [10,24].It's worth noting that from Fig. 4a, with the increase of SO 2 concentration in the flue gas, a decrease of SO 2 conversion is observed, which is consistent with the conclusion in the previous studies [5,10,12,24].As shown in Fig. 4b, ln(P SO 3 ) is plotted against ln(P 0 SO 2 ) , the linear fitting of the data point results in a slope of 0.98 under the conducted experimental parameters.The slope is close to unity which implies that the order of the reaction with respect to the SO 2 concentration is unity.In the previous work of Forzatti et al., where the oxidation of SO 2 was explored on a honeycomb catalyst, the results indicated that a first-order dependence could give a reasonable approximation of the SO 2 oxidation in the range from 0 to 1000 ppm of SO 2 [25,26].The reaction order obtained in present study is the SO 2 oxidation taking place in gas phase, which is different from the previous studies [25,26].

Effect of O 2 concentration
The effect of O 2 concentration on the SO 2 oxidation in gas phase was studied in a manner similar to that employed for studying the effect of the SO 2 concentration on the SO 3 formation.The conversion of SO 2 to SO 3 and the order of reaction with respect to the O 2 concentration were obtained.According to combustion condition in the tail gas incinerator of the sulfur recovery unit, experiments were performed at temperature 850 ℃ with a fixed SO 2 concentration of 1 vol% and by varying the O 2 concentration in 1, 2, 3, 4 and 5 vol%.
As shown in Fig. 5a, the concentration of SO 3 at the outlet of the reactor is plotted against the input O 2 concentration.As anticipated, the oxidation of SO 2 is enhanced with the enrichment of oxygen, due to more molecular oxygen existing in the flue gas.As O 2 concentration increases from 1 vol% to 2 vol%, the concentration of SO 3 increases significantly from 28 to 34 ppm.Further increasing O 2 concentration from 4 vol% to 5 vol% does not result in significant increase of SO 3 formation, which is consistent with the previous work from Fleig et al. [4].Different form the statement from previous work that at least 1% excess O 2 is needed for SO 2 oxidation [27], the conversion of SO 2 in the atmosphere of low excess O 2 is observed.In Fig. 5b, ln(P SO 3 ) is plotted against ln(P 0 O 2 ) , and the straight line fitted through the data points results in a slope of 0.25 under the conducted experimental conditions.According to the slope of the fitting data, the reaction order for the SO 2 oxidation with respect to the O 2 is 0.25, which is similar to the research of Fan et al. [28].The fractional order implies that the reaction between SO 2 and O 2 is not an elementary reaction.

Effect of experimental temperature
Under the standard-state conditions (i.e., 25 ℃ and 1 atm), the change in Gibbs free energy of the reaction 1 is − 71 kJ/mol [29].Therefore, thermodynamically, the oxidation of SO 2 in gas phase is feasible.However, the chemical kinetics of reaction 1 limits the conversion of SO 2 .The oxidation of SO 2 is too slow at low temperature.According to the thermodynamic model [18], thermodynamically, complete oxidation of SO 2 is expected at below 500 °C and, as the temperature is increased to 900 and 1000 °C, the conversion decreased to about 6.75 and 3.21%, respectively.To get a deeper insight into the temperature effect, the temperature dependence of the oxidation of SO 2 to SO 3 was investigated using a reactant mixture consisting of 1 vol % SO 2 , 4 vol % O 2 and by varying the experiment temperature among 600, 700, 800, 900 and 1000 ℃. Figure 6 shows the result of the temperature variation, and a positive correlation was found between the SO 2 oxidation and temperature.The concentration of SO 3 in the flue gas increased from 13 to 101 ppm as the temperature increased from 600 to 1000 ℃ in the performed experiments.It's worth noting that water vapor is absent in the tube reactor in the experiments conducted, and the O radicals combine with SO 2 corresponding to the oxidation of SO 2. The conversion of SO 2 is 0.13% at 600 ℃.
Higher concentration of O radicals at high temperature contributes to the SO 2 oxidation which results in conversion of 1% at 1000 ℃.This is consistent with the finding by Bayless [27].

Effect of water vapor
Generally speaking, the presence of moisture is inevitable in the flue gas in the tail gas incinerator.To evaluate the effect of moisture content on the oxidation of SO 2 in gas phase, the concentration of SO 3 at the outlet of the reactor was measured at water vapor contents varying form 0 vol% to 8.7 vol% in the flue gas.As shown in Fig. 7, the introducing of moisture in the reactor results in a significantly enhance of SO 2 oxidation.The SO 3 concentration at the outlet of the reactor is increased from 41 to 85 ppmv with the presence of water vapor.
(5) The water vapor exist in the flue gas increases the formation of H radicals through reaction 5 and thereby the formation of O and OH radicals by reaction 6.Furthermore, the concentration of OH radicals is increased through reaction 7.The increasing concentration of OH radicals enhances the oxidation of SO 2 by reaction 2. However, in the range of moisture content from 0.3% to 8.7%, all the measurements show comparable values, and no obvious dependency of the SO 2 oxidation on the water vapor content is visible.In the literature from Tobias and Belo [10,24], a similar trend was observed with the increase of moisture content.It's worth noting that due to the need to condense the SO 3 in the flue gas, all the experiments in the reference literature were conducted with the presence of moisture in the reactor.Due to the introducing of water vapor at outlet of the reactor in the present work, the experiments performed under conditions without water vapor in the reactor are feasible.The obvious enhancing effect of water vapor for SO 2 oxidation in gas phase was demonstrated in this study.Further work is needed to elucidate the independence of the varying moisture content for the SO 2 conversion in gas phase.

Effect of CO 2
Due to the considerable partial pressure of CO 2 in the tail gas incinerator of the sulfur recovery unit, the effect of CO 2 on the oxidation of SO 2 is considered.Figure 8 shows the SO 2 conversion varying with CO 2 concentration.As can be seen in Fig. 8, the increasing content of CO 2 has a very slight effect on the SO 2 oxidation at experimental temperature.The presence of CO 2 can depress the availability of O/H radicals, since the reaction between CO 2 and H radicals, as shown in reaction 8, is in competition with reaction 6.Therefore, the concentration of H radical can be decreased in the presence of high CO 2 concentration.The H radical is important to the production of O radicals and OH production.Thus, the SO 2 conversion would be decreased in the presence of CO 2 .However, on the other hand, the third body efficiency of reactions 1 and 2 for CO 2 is higher than that for N 2 , which counteracts the depressed effect occurring through radical removal by CO 2 [30].To conclude, the presence of CO 2 in the flue gas influences the SO 2 oxidation in both directions (production/reduction).Overall, no significant effect of CO 2 was found to the oxidation of SO 2 .This result is consistent with previous [12] works [5].

Conclusions
The homogeneous oxidation of SO 2 to SO 3 was investigated using a quartz flow reactor under the combustion conditions similar to those in the tail gas incinerator of the sulfur recovery unit.The conversion of SO 2 decreased with the increase of SO 2 concentration.The reaction order of SO 3 formation was determined to be 0.98 and 0.25 with respect to SO 2 and O 2 .The SO 2 /SO 3 conversion increased from 0.13% to 1% as the temperature increased from 600 to 1000 ℃.The enhancing of SO 2 conversion is the result of higher concentration of O radicals at high temperature.The presence of water vapor has a significant effect to promote the conversion of SO 2 .SO 2 oxidation was also found to be independent of the CO 2 in the flue gas.
Experiments were designed to determine the effect of different combustion conditions on SO 3 formation.The results of this research would provide an idea to reduce the production of SO 3 in tail gas combustor (such as the Claus installations).In the future, the effect of the combustion chamber structure combine with the operating parameter of combustor on the SO 3 formation maybe the research direction.

Fig. 1
Fig. 1 Experimental setup for homogeneous SO 2 oxidation experiments

Fig. 4 a
Fig. 4 a Effect of the input SO 2 concentration on the output SO 3 concentration in the flue gas and conversion of SO 2 to SO 3 , b order of dependency with respect to SO 2 concentration for SO 3 formation reaction.Inlet gas mixture for the experiments: variable ppm of SO 2 , 4 vol% O 2 , and balance N 2 , at temperature of 850 ℃

Fig. 5 aFig. 6
Fig. 5 a Effect of the input O 2 concentration on the output SO 3 concentration in the flue gas, b order of dependency with respect to O 2 concentration for SO 3 formation reaction.Inlet gas mixture for the experiments: variable ppm of O 2 , 1 vol% SO 2 , and balance N 2 , at temperature of 850 ℃

Fig. 7 Fig. 8
Fig. 7 Effect of the input moisture on the output SO 3 concentration in the flue gas and the conversion of SO 2 .Inlet gas mixture for the experiments: variable concentration of H 2 O, 1 vol% SO 2 , 4 vol% O 2 and balance N 2 , at temperature of 850 ℃